The use of magnetic devices comprising at least two layers made of a magnetic material separated by an interlayer made of a non-magnetic material is known in the field of magnetic memories and magnetic recording media. Generally speaking, these various layers and their interfaces are flat and parallel to each other.
In order to enhance the capabilities of such magnetic devices, attempts are made to make the structures of these memories increasingly compact. To achieve this, the magnetic devices use ferromagnetic layers that have ever smaller dimensions. In particular, these layers are becoming increasingly thinner and the stacks that form these magnetic devices are increasingly short and narrow.
By definition, the thickness or thinness of a layer is defined as the direction in which the various layers are stacked, i.e. the direction being perpendicular to the plane of the layers.
In the context of applications involving magnetic recording media or memories, it is important that each of the stacked magnetic layers is magnetically independent of the other magnetic layers.
If the magnetic device has layers that have large lateral dimensions, magnetic interactions (due to the magnetic field radiated by each of the magnetic layers) between the various layers that constitute the stack remain negligible.
However, if the magnetic structure is produced by using vapor deposition (techniques referred to as PVD, such as cathode sputtering for instance), magnetostatic charges may occur and build up at the level of the interfaces between the non-magnetic interlayer and each of the magnetic layers. The greater the surface roughness of the interfaces, the more numerous these magnetostatic charges will be.
In particular, if the non-magnetic layer has a constant thickness but has an average position that varies according to the direction in which the layers are stacked, the interlayer then has a “corrugated” shaped cross-section. The term “correlated roughness” is used in this case. If the interlayer has this kind of topology, the magnetic charges that occur induce a magnetic coupling field between the magnetic layers located either side of the non-magnetic interlayer.
In the case of magnetic layers with planar magnetization (that is when the direction of magnetization is parallel to the layer plane), it is known that this coupling field tends to align the magnetizations of the magnetic layers parallel to each other. The magnetic layers located either side of the non-magnetic interlayer are then no longer magnetically independent of each other. This coupling is generally referred to as Neel “orange-peel coupling” and, as a first approximation, does not depend on the lateral dimensions of the stack.
Moreover, when one miniaturizes the magnetic device by reducing the lateral dimensions of the stack down to just a few dozen or hundreds of nanometers, the fields radiated by each of the ferromagnetic layers, which are still assumed to have planar magnetization, create magnetostatic interaction between these layers.
By analogy with two magnetized bars that attract each other when they are moved closer together, the South pole of the first bar is preferentially attracted to the North pole of the second bar and vice versa, as shown schematically in FIG. 1a. This magnetostatic interaction therefore tends to orientate the magnetizations of the magnetic layers in an antiparallel fashion.
Consequently, the effects of this magnetostatic interaction, the force of which depends on the lateral dimensions of the stack, are in opposition to the effects of the coupling field due to the correlated roughnesses of the interlayer, thanks to the opposite signs of the respective magnetostatic fields that are created.
Thus, by carefully adjusting the intensity of these two effects, and by deliberately modifying, for example, the roughness of the interlayer, it is possible to produce a relatively compact magnetic recording medium, the magnetic layers of which are virtually magnetically independent of each other. Documents U.S. Pat. No. 6,727,105 and US-A-2004/013880 describe magnetic recording media made in this way in which undesirable magnetic effects mutually compensate each other.
In contrast, as explained in Document FR-A-2 859 306, there is an obstacle to the miniaturization of magnetic media with planar magnetization which complicates or prevents information being written by currently available write heads. This is the superparamagnetic limit, i.e. the limit that corresponds to lateral sizes below which magnetization of the system no longer remains stable, typically for a period of ten years at ambient temperature.
As stated in Document FR-A-2 859 306, magnetic devices with layers having a magnetization orientation perpendicular to their plane have a superparamagnetic limit that diminishes as sizes drop because the demagnetizing field reduces with greater miniaturization of the magnetic device.
In addition, write heads are currently more efficient, in terms of field amplitude produced and spatial resolution, for perpendicular recording rather than for planar recording. This is why attempts are currently being made to develop recording media with layers having magnetization perpendicular to the plane of said magnetic layers.
Just like devices with planar magnetization, devices with magnetization perpendicular to the plane of their magnetic layers are also affected by ferromagnetic interaction associated with the correlated roughness of the interlayer. This ferromagnetic interaction therefore creates a ferromagnetic coupling field between the magnetic layers located either side of the non-magnetic interlayer.
If the lateral dimensions of the device with perpendicular magnetization are large, the fields radiated by the magnetic layers located on one side of the interlayer cause negligible interaction with the magnetic layers located on the other side. Nevertheless and in the same way as in the case of magnetic devices with planar magnetization, if the lateral dimensions of the stacked layers are reduced, the field radiated by one magnetic layer then causes non-negligible magnetostatic interaction with the other magnetic layers.
On the other hand, unlike devices with planar magnetization, the coupling field due to this magnetostatic interaction is ferromagnetic rather than antiferromagnetic, as shown schematically in FIG. 1b. Consequently, roughness interaction (orange-peel coupling) and magnetostatic interaction no longer compensate each other but are added to each other, so that the magnetic layers located either side of the interlayer can no longer be made magnetically independent of each other by these two forms of coupling cancelling each other out.
FIG. 2 shows the magnetic hysteresis loops or magnetization curves of two multilayer magnetic structures having large lateral dimensions, i.e. structures between which there is hardly any magnetostatic interaction at all.
In this case, the stack that forms the magnetic device comprises two superposed multilayers made of cobalt and platinum, separated by a thick non-magnetic interlayer of platinum having the formula (Pt1.8/Co0.6)2/Pt15/(Co0.6/Pt1.8)4. The numbers 1.8, 0.6 and 15 denote the thicknesses, in nanometers, of the chemically homogeneous layers (designated by their chemical elements) to which they are attached. Coefficients 2 and 4 denote the number of times the (Co/Pt) or (Pt/Co) multilayer pattern repeats either side of the interlayer.
For this stack, bearing in mind the small thickness of the layers used (in this example 1.8 and 0.6 nm for the platinum and cobalt), each multilayer can be regarded as a magnetically homogeneous layer. The use of these multilayers makes it possible to ensure that the magnetization of each of them is perpendicular to the plane of the layers, something which would not be the case, for example, in the case of a pure cobalt layer.
These (Co/Pt) multilayers could be replaced by any other material, alloy or multilayer that also has the property of magnetization perpendicular to its plane, and are merely used in order to illustrate the physical phenomena that are exploited. (Co/Pd) multilayers or chemically ordered alloys FePt or FePd could also be used, for example. Each of the two multilayers is referred to as a “magnetic layer” in the rest of this document.
Similarly, the nature of the interlayer is not confined to platinum and any other non-magnetic material, alloy or multilayer can be used, provided that this interlayer preserves the structural integrity of the stack and orientation of the magnetizations of the various magnetic layers perpendicular to their plane. Its thickness depends on the material used and will be chosen to ensure that any magnetic coupling between the two magnetic layers through this interlayer is far less intense than the coupling between the layers of cobalt inside each multilayer.
In the magnetic hysteresis loops shown in FIG. 2, the arrows indicate the direction of the magnetic hysteresis loop when magnetic excitation H indicated on the x-axis is applied. The unidirectional arrows represent irreversible loop changes and the bidirectional arrows represent reversible loop changes. The magnetization levels are indicated in arbitrary units on the y-axis.
The dotted line curves correspond to a so-called major magnetic hysteresis loop, i.e. a loop for which the amplitude of the variation in the magnetic excitation field is sufficient to successively reverse the magnetization direction of the two magnetic layers, whereas the curves shown a solid line correspondent to a so-called minor magnetic hysteresis loop in which the smaller amplitude of the applied magnetic field only results in reversal of the softer magnetic layer.
Magnetic “hardness” (or “softness”) is defined in relation to the value of the magnetic field called the coercive field that it is required in order to reverse the magnetization of each of the magnetic layers. A particular magnetic layer will therefore be said to be “hard” if its coercive field is relatively high.
If H+ and H− denote the reversal fields of the softer magnetic layer for applied increasing and decreasing magnetic fields respectively (see FIG. 2), the value of the coercive field of this layer is given by:Hc1=|H+−H−|/2where the symbols ∥ denote the absolute value of the difference in the reversal fields. Points 1 and 2 in FIG. 2 correspond to the situation where magnetization of the “harder” layer points upwards (positive magnetization), whereas that of the “softer” magnetic layer points either upwards (1) or downwards (2).
Similarly, points 3 and 4 correspond to the situation where magnetization of the “harder” layer points downwards (negative magnetization), whereas that of the “softer” magnetic layer points either upwards (3) or downwards (4). The coercive field of the softer magnetic layer can therefore also be determined, as above, when the magnetization of the “harder” magnetic layer is negative.
If the minor magnetic hysteresis loop is symmetrical relative to the zero magnetic field, |H+|=|H−|, indicating the magnetic independence of the two layers. In contrast, displacement of the so-called minor magnetic hysteresis loop relative to the zero magnetic field reveals interaction between the two magnetic layers through the non-magnetic interlayer, this displacement being either towards positive fields or negative fields, depending on the sign of the interaction.
Using the same definitions as those used above, this coupling field between the two magnetic layers Hcpl can be calculated as follows:Hcpl=(H++H−)/2
Interaction will be referred to as ferromagnetic if Hcpl is negative (parallel magnetizations) and antiferromagnetic if Hcpl is positive (antiparallel magnetizations).
In FIG. 2 one can see that the magnetic hysteresis loop of the softer magnetic layer is virtually centered relative to zero magnetic excitation fields (H=0 kOe). The slight offset of 5 Oe towards negative fields indicates the presence of slight surface roughness of the interfaces tending to stabilize parallel orientation of the magnetizations of the two layers, as was stated previously. If one ignores this ferromagnetic coupling which is weak compared to the values of the coercive fields, the two magnetic layers either side of the interlayer are therefore essentially magnetically independent of each other.
In a known manner, in the case, for instance, of applications involving storage memory type devices, when one uses two layers of different magnetic hardness either side of the interlayer, one can determine four stable states with a zero excitation field and this makes it possible to double the quantity of information stored compared with using only one magnetic layer, which has only two stable zero-field states.
The interlayer that separates the magnetic layers, in this example made of platinum, is relatively thick because it measures 15 nm. The compositions of each cobalt and platinum multilayer (layer thicknesses, number of repetitions of the basic Co/Pt pattern), result in coercive fields Hc1 and Hc2 of the two magnetic layers that have very different values.
As explained above, magnetostatic interaction between the magnetic layers is negligible in this case because the lateral dimensions of the stack are large and the interlayer is thick. There only remains the small above mentioned contribution made by the surface roughness of the interfaces.
A magnetic device comprising layers with perpendicular magnetization as defined above therefore has four magnetic stable zero-field states, i.e. when no magnetic field is applied to the device. These four magnetic states are, respectively, the up-up, up-down, down-up and down-down states and correspond to points 1, 2, 3 and 4 in FIG. 2.
If M1 and M2 denote the magnetizations of each of the two magnetic layers, with M2 denoting, for example, magnetization of the magnetically harder layer, the resulting magnetization corresponding to each of these states will be (M2+M1), (M2−M1), (−M2+M1) and (−M2−M1) for states 1, 2, 3 and 4 in FIG. 2. It is apparent that only 3 distinctive states will be possible when M2=M1, because in this case states 2 and 3 will be equivalent in terms of total magnetization.
In order to achieve each of these four stable states, when M1 is different to M2, the magnetic device must be subjected to the following sequences of magnetic excitation fields (the numeric values stated correspond to the case envisaged in FIG. 2 and obviously depend on the values of the coercive fields Hc1 and Hc2 of the two magnetic layers and hence the particular structure of each of the magnetic layers):                for state 1, H=+0.6 kOe, then H=0 kOe;        for state 2, H=+0.6 kOe, then H=−0.3 kOe and finally H=0 kOe;        for state 3, H=−0.6 kOe, then H=+0.3 kOe and finally H=0 kOe;        for state 4, H=−0.6 kOe, then H=0 kOe;        
The discussion above was based on a type of application referred to as “multi-coercive field magnetic storage” in which the four distinctive magnetization states make it possible to double the density of information stored compared with a conventional memory comprising a single magnetic layer that therefore only has two magnetizations states (up or down).
In another type of application, for instance non-volatile magnetic memories or reprogrammable logic gates, one uses stacks of magnetic tunnel junction type or spin-valve type layers that also consist of two magnetic layers separated by a non-magnetic metal or insulating layer intended to ensure magnetic independence of the two magnetic layers.
In this case, the memory state is read by measuring its electrical resistance by causing an electric current to flow in a direction that is perpendicular to the plane of the layers. This electrical resistance is higher when the magnetizations of the two layers are antiparallel rather than parallel. This phenomenon is referred to as “giant magnetoresistance” if the non-magnetic separating layer is metallic or as “tunnel magnetoresistance” if the non-magnetic separating layer is an insulator and, as is well-known in the literature, only depends on the relative orientation of the magnetization of the two layers.
In FIG. 2, states 1 and 4 are therefore impossible to discern, as are states 2 and 3, because they correspond to the same relative magnetization orientation (parallel for states 1 and 4, antiparallel for states 2 and 3). These states will be referred to as “degenerate states”. In any application where the wanted signal is the electrical resistance of the stack (e.g. memories, logic gate), the information used is not the number of magnetic states but only the number of degenerate states, which is half as high.
It nevertheless remains true that, in order for such a memory or logic gate to function, it is still necessary that the two degenerate states (1, 4) and (2, 3) that correspond to parallel and antiparallel magnetization directions of the two magnetic layers respectively are both stable in a zero magnetic field, i.e. when the device is not subjected to any external magnetic field. Compared with the above discussion, it is therefore necessary that the coupling field between layers is strictly less than the coercive field of the “softer” magnetic layer.
FIG. 3 is a schematic representation of two magnetic layers with magnetization perpendicular to their plane, separated by a non-magnetic layer. M1 and M2 are the magnetizations of the two layers, the outline arrows represent the direction of the magnetizations of these layers and the solid arrows schematically show the magnetic fields radiated from one layer to the other and tending to align the magnetizations of the two magnetic layers parallel to each other. Reducing the lateral dimensions causes positive and negative magnetostatic charges to appear at the interfaces of the magnetic layers, giving rise to magnetostatic coupling that encourages parallel alignment of the magnetizations of the two magnetic layers.
FIG. 4 illustrates the effect of reducing the lateral dimensions of a magnetic device such as that described in relation to FIG. 3 on the curve that shows the variation in magnetization as a function of the applied magnetic field. In this case, as explained above, a ferromagnetic type coupling field appears between the magnetic layers located either side of the interlayer due to the effect of magnetostatic interaction.
Consequently, as shown in FIG. 4, the magnetization curve of the softer magnetic layer (continuous line) is displaced relative to the zero magnetic field on the y-axis towards negative magnetic fields. The magnetizations of the two layers tend to remain parallel to each other as long as possible due to the effect of this ferromagnetic coupling. It will then be necessary to apply a magnetic field H− having a greater amplitude than in the case of FIG. 2 (equal to the total of coercive field Hc1 and coupling field Hcpl) in order to reverse the magnetization of the soft layer.
Moreover, if the ferromagnetic coupling field that causes this displacement exceeds coercive field Hc1 of the softer magnetic layer, as is the case in FIG. 4, the intermediate up-down and down-up states (points 2 and 3 in FIG. 2) will no longer be stable in a zero magnetic field. As is evident from FIG. 4, only the two extreme up-up and down-down states that correspond to points 1 and 4 respectively are capable of being stable once one ceases to subject the magnetic device to an excitation field such as that generated by a write head.
Consequently, using such a magnetic recording medium, the quantity of information that can be recorded is halved compared with the device described in relation to FIG. 2.
Because such a stack has two magnetic states that are stable in a zero field (up-up and down-down) that both correspond to the same relative orientation of the magnetizations of the two magnetic layers, it has no functional application for devices such as non-volatile memories or logic gates because the only state that is stable in a zero magnetic field will be a degenerate state that corresponds to the situation where the magnetizations are parallel (low-resistance state).
It will nevertheless be possible to access the antiparallel state (high resistance) by means of a magnetic field pulse, but this state will not be maintained once the magnetic field pulse has decayed and non-volatility will be lost.